U.S. patent number 9,094,875 [Application Number 13/896,032] was granted by the patent office on 2015-07-28 for base station, mobile station, communication system, transmission method and reordering method.
This patent grant is currently assigned to FUJITSU LIMITED. The grantee listed for this patent is FUJITSU LIMITED. Invention is credited to Yoshihiro Kawasaki, Kazuhisa Obuchi, Yoshiaki Ohta, Katsumasa Sugiyama, Yoshiharu Tajima, Yoshinori Tanaka.
United States Patent |
9,094,875 |
Ohta , et al. |
July 28, 2015 |
Base station, mobile station, communication system, transmission
method and reordering method
Abstract
A mobile station that includes a receiving means for receiving
first PDCP (Packet Data Convergence Protocol) PDU (Protocol Data
Unit) to which a first sequence number is added from a source base
station; a receiving means for receiving second PDCP PDU from a
target base station that applies a second sequence number
transferred from the source base station as a sequence number of
PDCP (Packet Data Convergence Protocol) SDU (Service Data Unit)
received from a host station and creates the second PDCP PDU and a
storage means for storing PDCP SDU corresponding to the first PDCP
PDU and PDCP SDU corresponding to the second PDCP PDU and means for
performing in order delivery of the stored PDCP SDUs based on the
sequence numbers corresponding to the PDCP SDUs.
Inventors: |
Ohta; Yoshiaki (Kawasaki,
JP), Tajima; Yoshiharu (Kawasaki, JP),
Obuchi; Kazuhisa (Kawasaki, JP), Kawasaki;
Yoshihiro (Kawasaki, JP), Tanaka; Yoshinori
(Kawasaki, JP), Sugiyama; Katsumasa (Kawasaki,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITSU LIMITED |
Kawasaki-shi, Kanagawa |
N/A |
JP |
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Assignee: |
FUJITSU LIMITED (Kawasaki,
JP)
|
Family
ID: |
39943237 |
Appl.
No.: |
13/896,032 |
Filed: |
May 16, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130250862 A1 |
Sep 26, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12603125 |
Oct 21, 2009 |
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PCT/JP2007/059037 |
Apr 26, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W
36/023 (20130101); H04L 49/9057 (20130101); H04L
47/10 (20130101); H04L 47/14 (20130101); H04W
36/02 (20130101); H04W 28/10 (20130101); H04L
47/34 (20130101); H04W 36/0033 (20130101) |
Current International
Class: |
H04W
4/00 (20090101); H04L 12/801 (20130101); H04L
12/861 (20130101); H04W 36/02 (20090101); H04W
36/00 (20090101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1669350 |
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Sep 2005 |
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CN |
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1669350 |
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1059823 |
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EP |
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Jun 2005 |
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1545143 |
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Jun 2005 |
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EP |
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2004-282652 |
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Oct 2004 |
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JP |
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2006-5949 |
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Jan 2006 |
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2007-006230 |
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Jan 2007 |
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2007-6230 |
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Dec 2005 |
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WO |
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2006/130354 |
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Dec 2006 |
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WO |
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2006/135185 |
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Dec 2006 |
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WO |
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Primary Examiner: Moore, Jr.; Michael J
Attorney, Agent or Firm: Myers Wolin, LLC
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a division of U.S. application Ser. No.
12/603,125, filed on Oct. 21, 2009, now pending, which is a
continuation of PCT Application of PCT/JP2007/059037, filed on Apr.
26, 2007, the contents of each are herein wholly incorporated by
reference. The present application also relates to U.S. application
Ser. No. 13/430,207, filed Mar. 26, 2012, U.S. application Ser. No.
13/894,062, filed May 14, 2013 and U.S. application Ser. No.
13/894,766 filed May 15, 2013.
Claims
What is claimed is:
1. A mobile station comprising: a receiver configured to receive a
first PDCP (Packet Data Convergence Protocol) PDU (Protocol Data
Unit) to which a first sequence number is added from a source base
station and a second PDCP PDU from a target base station that
applies a second sequence number transferred from the source base
station as a sequence number of a PDCP (Packet Data Convergence
Protocol) SDU (Service Data Unit) received from a host station and
creates the second PDCP PDU; a buffer configured to store a PDCP
SDU corresponding to the first PDCP PDU and a PDCP SDU
corresponding to the second PDCP PDU; and a reordering unit
configured to perform in order delivery of the stored PDCP SDUs
based on the sequence numbers corresponding to the PDCP SDUs.
2. A radio communication system, comprising: a source base station;
a target base station; and a mobile station wherein the mobile
station including comprises: a receiver configured to receive a
first PDCP (Packet Data Convergence Protocol) PDU (Protocol Data
Unit) to which a first sequence number is added from the source
base station and a second PDCP PDU from the target base station
that applies a second sequence number transferred from the source
base station as a sequence number of a PDCP (Packet Data
Convergence Protocol) SDU (Service Data Unit) received from a host
station and creates the second PDCP PDU; a buffer configured to
store a PDCP SDU corresponding to the first PDCP PDU and a PDCP SDU
corresponding to the second PDCP PDU; and a reordering unit
configured to perform in order delivery of the stored PDCP SDUs
based on the sequence numbers conforming to the PDCP SDUs.
3. A base station, comprising: a receiver configured to receive a
first sequence number from a source base station; a control unit
configured to apply the first sequence number as a sequence number
of a PDCP (Packet Data Convergence Protocol) SDU (Service Data
Unit) received from a host station and creates a first PDCP (Packet
Data Convergence Protocol) PDU (Protocol Data Unit); and a
transmitter configured to transmit the first PDCP PDU to a mobile
station which receives a second PDCP PDU to which a second sequence
number is added from the source base station, stores a PDCP SDU
corresponding to the first PDCP PDU and a PDCP SDU corresponding to
the second PDCP PDU and performs in order delivery of the stored
PDCP SDUs based on the sequence numbers corresponding to the PDCP
SDUs.
Description
BACKGROUND OF THE INVENTION
The present invention relates to abase station, mobile station,
communication system, transmission method and reordering
method.
Currently, for mobile communication systems such as mobile
telephones, third-generation type service using CDMA has begun,
however, research of a next generation mobile communication system
(LTE: Long Term Evolution) in which even faster communication is
possible is being advanced by the 3GPP (3rd Generation Partnership
Project) (refer to 3GPP, "Requirements for Evolved UTRA (E-UTRA)
and Evolved UTRAN (E-UTAN), " TR25.913 V7.3.0, Release 7, March
2006). Some of the large challenges in this research include
increasing the transmission rate and reducing transmission
delay.
In the LTE communication system, in order to increase the
transmission rate and reduce transmission delay, the handover
method used is designed to be at a higher level than that of a
conventional system. In a mobile communication system, when a
mobile station moves during communication, the base station that
the mobile station is communicating with is switched (handover)
according to the reception status. Therefore, in order to perform
communication at increased transmission speed and with low
transmission delay, improving the level of the handover is
essential. In a LTE communication system, a packet exchange system
is basic, so handover is a hard handover. In a hard handover, after
the line connection with the base station that the mobile station
communicates with before moving is cut, a line is connected between
the mobile station and a target base station. In a hard handover,
by obtaining system information about the target base station
immediately before performing the handover, it is possible to
perform the handover in a short time; however, transmission of user
data becomes interrupted during the handover.
Therefore, in order to reduce transmission delay it is important
that the state of interrupted transmission be shortened and that
the loss of packets while transmission is interrupted be prevented.
In the case that packets do become lost while transmission is
interrupted, the lost packets are recovered in end-to-end
retransmission of the packets, so transmission delay becomes
large.
Therefore, in a handover in a LTE communication system, a method is
specified in which among data that includes control information and
packets for the mobile station, the transmission of packets that
have not yet been transmitted to the mobile station from the source
base station is taken over by forwarding those packets to the
target base station from the source base station (refer to 3GPP,
"Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved
Universal Terrestrial Radio Access Network (E-UTRAN)," TS36.300,
Release 8, V8.0.0, April 2007).
Takeover During a Handover
FIG. 21 is a drawing explaining takeover during a handover. In (A)
of FIG. 21, two base stations 1a and 1b are connected to a host
station (for example a MME/SAE gateway) 2. A mobile station 4
exists within the cell 3a of the base station 1a, and is currently
communicating with the base station 1a. In this state, as shown in
(B) of FIG. 21, when the mobile station 4 moves in the direction
toward the base station 1b and enters into the cell 3b, a handover
is executed and the base station with which the mobile station 4
communicates is switched from the base station 1a to the base
station 1b. Here, a base station that is in communication with a
mobile station before the mobile station moves is called the source
base station, and the base station that communicates with the
mobile station after the mobile station moves is called the target
base station.
The source base station 1a stores packets that were received from
the host station 2 in an internal buffer, and sequentially sends
the packets that are stored in that buffer to the mobile station 4.
Therefore, when a handover occurs, the packets that are not sent to
the mobile station are stored and exist in the buffer. In (B) of
FIG. 21, it is necessary that before a handover, the packets n-2 to
n be received and stored in the buffer without being sent to the
mobile station, and that after the handover it is necessary that
these packets be sent to the mobile station 4 from the target base
station. Therefore, when executing the handover sequence, the
source base station 1a transfers (forwards) the packets n-2 to n to
the target base station 1b. By using this forwarding method, the
target base station sends those packets to the mobile station 4
immediately after the handover, so no interruption in the packets
occurs. Therefore, it becomes possible to execute high-speed
handover without performing end-to-end retransmission of the
packets. In the explanation above, n-2 to n are numbers (sequence
numbers) that indicate the order of the packets.
Handover
FIG. 22 is a drawing explaining a handover in a LTE communication
system, and FIG. 23 is a drawing explaining the handover procedure
that is currently presumed for a LTE communication system.
Using a Measurement Report (report of the reception status of the
base station 1 and other surrounding base stations), the mobile
station 4 notifies the source base station 1 that a handover HO
(Handover) is necessary (1. Measurement Control).
The source base station 1 decides a target base station 1b
according to the contents of the Measurement Report (2. HO
Decision), and sends a handover request (3. Handover Request) to
that target base station 1b. At that time, the source base station
1a also sends information about the mobile station (mobile station
ID, QoS (Quality of Service) information, etc.). The target base
station 1b performs call-receiving control based on that
information (4. Call-Receiving Control).
After allowing the acceptance of the mobile station, the target
base station 1b returns a handover response to the source base
station (5. HO Response). After that, the source base station 1a
sends a handover instruction to the mobile station 4 (6. HO
Instruction), then immediately afterwards starts taking over the
data (packets) (Packet transfer: Forwarding).
The mobile station 4 receives the handover instruction, and then
obtains synchronization using L1/L2 signaling (7. Synchronization),
and after synchronization has been obtained, sends a handover
complete report to the target base station 1b (8. HO Complete).
After this, the target base station 1b sends a handover complete
report to the host station 2 (9. HO Complete). After receiving the
handover complete report, the host station 2 changes the packet
transmission path from the source base station 1a to the target
base station 1b (10. Path Change), and returns a HO complete
response to the target base station 1b (11. HO Complete Response).
The target base station 1b uses a HO complete response to notify
the source base station 1a that the handover HO is complete (12.
Resource Release). After that, the path between the source base
station 1a and the host station 2 is eliminated (13. Resource
Release).
Packet Order Alignment Control
When packets are forwarded (forwarded) during execution of the
handover sequence described above, there is a possibility that
packets that are transferred by the target base station 1b will be
jumped over by the packets that flow from the host station 2
resulting in the sequence numbers becoming mixed up. When packets
are transferred from the target base station 1b to the mobile
station 4 as are with the sequence numbers mixed up, the mobile
station 4 is unable to receive the packets in the correct order, so
the quality of communication is degraded, and as a result
high-quality communication cannot be achieved just before and after
a handover.
Therefore, in the LTE communication system, by using a method as
described below, the base station and mobile station are capable of
maintaining packet order. FIG. 24 is a drawing explaining packet
order alignment wherein the target base station 1b maintains the
packet order by sending packets that are transferred from the
source base station 1a with higher priority than packets that are
received from the host station.
In FIG. 24, before a handover, packets n-5 to n are stored in the
source base station 1a, and of these packets, packets n-5 to n-2
are sent to the mobile station 4, however, packets n-1 and n are
not sent to the mobile station 4. In addition, of the packets that
are sent to the mobile station 4, packets n-5 and n-3 are not
rightly received by the mobile station 4 (NACK), and packets n-4
and n-2 are rightly received (ACK). Therefore, the mobile station 4
save the packets n-4 and n-2 in a buffer BF1, and does not save the
packets n-5 and n-3.
When a handover occurs in this state, the source base station 1a
transfers (forwards) the packets n-5 and n-3, which were not
rightly received by the mobile station 4, and the packets n-1 to n,
which have not yet been sent to the mobile station 4, to the target
base station 1b, and the target base station 1b stores those
packets in a buffer BF. Also, after the handover, the host station
2 sends two packets m to m+1 that are intended for the mobile
station 4 to the target base station 1b, and the target base
station 1b stores those packets in the buffer BF.
After communication with the mobile station 4 becomes possible, the
target base station 1b first sends the packets n-5, n-3 and n-1 to
n that were forwarded from the source base station 1b to the mobile
station 4. Next, the target base station 1b sends the packets m to
m+1 that were received from the host station 2. As shown in FIG.
25, the mobile station 4 rearranges the sequence order of the
packets n-4 and n-2 that were received before the handover, and the
packets n-5, n-3, n-1 and m to M+1 that were received after the
handover, and gives those packets in order to an upper layer.
In the explanation above, the case was explained in which all of
the packets n-5, n-3 and n-1 to n were transferred (forwarded) to
the target base station 1b, however, in some cases, only the
packets n-5 and n-3 will be transferred, and transferring of
packets n-1 to n will be delayed. FIG. 26 is a drawing explaining
packet order arrangement in that case. The target base station 1b
stores the transferred packets n-5 and n-3 together with the
packets m to m+1 that come from the host station 2 in a buffer BF,
however, sends the packets n-5 and n-3 that were forwarded from the
source base station 1a to the mobile station 4 first. After that,
in the case that packets n-1 and n are forwarded from the source
base station 1a later, the target base station 1b monitors whether
a set time (waiting time) has elapsed, and when the packets n-1 and
n still have not been forwarded from the source base station 1a
even after the waiting time has elapsed, the target base station 1b
determines that forwarding has been completed and sends the packets
m and m+1 that were received from the host station to the mobile
station to the mobile station. Even though the packets n-1 and n
may be received from the source base station 1a after forwarding
has been completed, the target base station 1b discards those
packets.
The mobile station 4 executes a process for rearranging the order
of the sequence numbers of the received packets (reordering). As
shown in FIG. 27, the mobile station 4 rearranges the sequence
numbers of the packets n-4 and n-2 that were received before the
handover, and the packets n-5, n-3, m and m+1 that were received
after the handover, and gives those packets in order to an upper
layer.
Protocol Configuration
As described above, in a handover in a LTE communication system,
packet transfer (forwarding) and packet reordering are necessary
techniques. The relationship between these functions will be
explained in more detail here.
FIG. 28 is a drawing explaining the protocol configuration between
a mobile station and a base station. Between a mobile station and a
base station there is at least a PDCP (Packet Data Convergence
Layer) layer, RLC (Radio Link Control) layer and a lower layer (MAC
layer/physical layer MAC/PHY). A packet routing function or the
like is provided in MME/S-GW.
The main features of each protocol are as described below.
(1) PDCP: In the PDCP layer, the transmitting side compresses the
upper protocol header, as well as attaches a sequence number and
performs transmission. The receiving side checks the sequence
number, and by doing so, performs a discarding process for
redundant reception. Retransmission is not performed in the PDCP
layer.
(2) RLC: The RLC layer is a layer having a retransmission function,
and in the RLC layer, a sequence number is newly attached to the
data that is different from the sequence number that is attached to
the data from the PDCP layer, and the data is then transmitted. For
example, when data having a sequence number n is received from the
PDCP layer, that data is divided into a plurality of data, and to
each division of data, sequence numbers I(1), I(2), I(3), . . . are
attached in the RLC layer, after which the data is transmitted. The
receiving side notifies the sending side by using those sequence
numbers I() to send a transmission confirmation (Ack/Nack signal)
indicating whether the data were received rightly or improperly.
When an Ack signal is returned, the sending side deletes the data
that is saved, however, when a Nack signal is returned, the sending
side retransmits the saved data.
(3) Lower Layer
MAC: The MAC layer is a layer that multiplexes/demultiplexes the
data of the RLC layer. In other words, the sending side multiplexes
the data of the RLC layer and transmits the data, and the receiving
side demultiplexes the received data of the MAC layer to RLC layer
data.
PHY: The PHY layer is a layer for transmitting and receiving data
by radio signals between the user terminal 4 and base station 1,
and converts MAC layer data to radio data, or converts radio data
to MAC layer data.
Data destined for a mobile station first flows from the upper layer
(for example IP layer) to the PDCP layer to become a PDCP SDU
(Service Data Unit), then header information (PDCP layer sequence
number, etc.) is attached to become a PDCP PDU (Protocol Data
Unit).
The PDCP PDU is sent to the RLC to become a RLC SDU, where header
information (RLC layer sequence number, etc.) is further attached
to become a RLC PDU. The RLC PDU passes through the lower layer
processing after which it arrives at the RLC layer of the mobile
station. In this RLC layer, the header is deleted and the RLC SDU
is reassembled, then in the PDCP layer, the PDCP PDU header is
deleted to become a PDCP SDU, then the data is sent to the upper
layer.
In this kind of protocol configuration, in a LTE communication
system, the forwarding of packets is performed in PDCP SDU units,
and reordering is performed in PDCP PDU units. When forwarding is
performed in PDCP SDU units, header information such as a sequence
number is not attached to the PDCP SDU unit packet, so that
sequence number is not forwarded.
Therefore, in a case where the forwarding is performed in PDCP SDU
units, it is necessary to forward PDCP SDU data and harder
information including the sequence number separately.
The RLC SDU data and PDCP PDU data are essentially the same data,
so in the specification for the present invention, unless
specifically specified, these will simply be called packets, and
when a packet number is provided, that number is the sequence
number of the PDCP PDU data.
Operation of the Source Base Station
FIG. 29 is a flowchart of the operation of a source base station
during a handover.
When the source base station 1a receives the reception field
strength from the user terminal 4 by way of a Measurement Report
(step 101), the source base station 1a determines whether or not a
handover HO is necessary (step 102), and when a handover is not
necessary, returns to the beginning.
However, when it is determined that a handover is necessary, the
source base station 1a decides a target base station 1b according
to the contents of the Measurement Report, and sends a handover
request to that target base station 1b (step 103).
After that, the source base station 1a receives a handover response
that is sent from the target base station 1b (step 104), and
forwards the remaining packets to a target base station (step
107).
After receiving a resource release message that is sent from the
target base station 1b (step 108), the source base station 1a
executes the resource release (step 109).
Operation of the Target Base Station
FIG. 30 is a flowchart of the operation of a target base station
during a handover.
After receiving a HO request from the source base station 1a
(including the mobile station ID, QoS information, etc.) (step
121), the target base station 1b performs call receiving control
based on that information, and determines whether to allow the
acceptance of the mobile station (step 122), and when the mobile
station is not allowed, performs post processing (step 130) and
ends handover control.
On the other hand, when acceptance of the mobile station is
allowed, the target base station 1b returns a HO response to the
source base station 1a (step 123). The target base station 1b then
stores the packets that are forwarded from the source base station
1a in a buffer (step 124), and receives a HO complete report from
the mobile station 4 (step 125). After receiving the HO complete
report, the target base station 1b sends a HO complete report to
the host station 2 (step 126). The host station 2 receives the
handover complete report, then changes the packet transmission path
from the source base station 1a to the target base station 1b, and
returns a HO complete response to the target base station 1b. After
receiving the HO complete response from the host station 2 (step
127), the target base station 1b starts sending the packets that
were forwarded from the source base station 1a to the mobile
station preferentially, and after those packets have been sent,
sends the packets that were received from the host station 2 to the
mobile station (scheduling: step 128). In addition, at the same
time as step 128, the target base station 1b sends a resource
release to the source base station 1a (step 129), then performs
post processing (step 130) and ends handover control. In the
scheduling process of step 128, when forwarding of packets from the
source base station 1a is delayed, the target base station 1b
monitors whether a set time (Waiting Time) has elapsed, and when no
packets have been transferred even though the Waiting Time has
elapsed, the target base station 1b determines that forwarding has
finished and sends all the packets that were received from the host
station 2, and even though packets may be received from the source
base station 1a after forwarding has finished, the target base
station 1b discards those packets.
Operation of a Mobile Station
FIG. 31 is a flowchart of the operation of a mobile station during
a handover.
The measurement unit of the mobile station 4 sends a notification
of the reception field strength or the like to the source base
station by way of a Measurement Report (step 151). After that, the
mobile station 4 waits for a HO instruction from the source base
station la, and after receiving a HO instruction (step 152),
obtains synchronization with the target base station 1b using L1/L2
signaling (step 153), and after synchronization has been obtained,
sends a handover complete report to the target base station 1b
(step 154), then, in the case where packets are received from the
target base station 1b, the mobile station 4 executes a reordering
process (steps 155 to 160).
In other words, when the control unit of the mobile station
receives lower layer packets from the target base station 1b, the
control unit creates RLC SDU data and delivers that RLC SDU (PDCP
PDU) data to a reordering unit (step 155). The reordering unit
checks whether there are any missing sequence numbers (step 156),
and when there are no missing sequence numbers and the sequence
numbers are continuous, delivers PDCP SDU data obtained by removing
a header from the RLC SDU (PDCP PDU) data to the upper layer (step
160). However, when there is a missing sequence number, the control
unit instructs the reordering unit to save the RLC SDU (PDCP PDU)
data. By doing so, the reordering unit saves the PDCP PDU data
(step 157), and checks whether RLC SDU (PDCP PDU) data having a
continuous sequence number has been received (step 158). When RLC
SDU (PDCP PDU) data having a continuous sequence number has been
received, the reordering unit delivers PDCP SDU data obtained by
removing a header from the RLC SDU (PDCP PDU) data to the upper
layer, a well as delivers PDCP PDU data obtained by removing a
header from the saved PDCP PDU data to the upper layer (step
160).
However, in step 158, when RLC SDU (PDCP PDU) data having a
continuous sequence number is not received, the reordering unit
monitors whether a preset amount of time has elapsed (step 159),
and when that set amount of time has not elapsed, repeats the
process from step 157; however, when the set time has elapsed, the
reordering unit delivers PDCP SDU data obtained by removing a
header from the saved PDCP PDU data to the upper layer even though
the sequence numbers may not be continuous (step 160).
Problems
In a LTE communication system, the following problems exist when
executing packet forwarding during a handover. That is, as was
described above, when a handover is executed in a LTE communication
system, a process for taking over the packet for the mobile station
that remains at the source base station 1a is executed, and packets
are forwarded to a target base station during that taking over
process. However, in the handover control described above, if the
value of the waiting time by the target base station 1b is small,
the target base station 1b begins sending the packets received from
the host station regardless of whether or not all of the packets
have been forwarded, so a problem occurs in that the packets that
have not yet been forwarded are discarded. On the other hand, if
the value of the waiting time is large, there is a problem in that
the target base station 1b cannot send the packets received from
the host station until the waiting time has elapsed even though all
of the packets may have already been forwarded, so a transmission
delay occurs. In other words, in conventional handover control
there is an increase in communication delays and throughput is
degraded, making it impossible to maintain high-quality
communication immediately before and after a handover.
As first related art is a method of notifying the target base
station of the last packet to be forwarded from the source base
station (refer to Samsung, "Method to Release Resources at Source
ENB During Handover," R3-061032, RAN3#53, September 2006). When
forwarding is delayed, the target base station that has been
notified of the last packet can transmit the packets received from
the host station by starting the sequence numbers of those packets
from the sequence number of the last packet+1. In addition, by
comparing the sequence numbers of the packets that are forwarded
with the sequence number of the last packet, it is possible to
forcibly end the waiting time and to detect the end of forwarding
with optimum timing. However, in the case that the last packet is
deleted during forwarding, the target base station will be unable
to accurately detect the last packet.
Moreover, as second related art is a mobile communication system
for making high-speed packet data transfer possible with no data
loss during a handover between base stations during high-speed
packet communication (refer to Japanese patent publication No.
JP2004-282652A). When a handover occurs in this mobile
communication system, the source base station of the handover
transfers (forwards) packet data to the target base station of the
handover. However, there is an increase in communication delay due
to reordering by the mobile station, and there is no improvement in
the degraded throughput.
Furthermore, as third related art is a method in which the target
base station jumps the packets sent from the host station without
waiting for forwarded packets to arrive (refer to Japanese patent
application No. 2006-086537A). In this method, by distinguishing
the packets that are received from the source base station from the
packets that are received from the host station, transmission is
possible in which the packets jump. However, it is necessary for
the mobile station to have two order control functions, and the
control thereof becomes complicated.
SUMMARY OF THE INVENTION
Taking into consideration the aforementioned problems, it is the
object of the present invention to quickly transmit packets that
are sent from a host station (for example, a device that is
different from a source base station, and that sends data (packets)
to a target base station) to a target base station to a mobile
station.
Another object of the present invention is to properly reorder
forwarded packets even when there is only one order-control
function and even when packets that are forwarded from a source
base station are mixed with packets that are sent from the host
station by excluding the packets that are sent from the host
station as the object of order control.
Providing items that are described in the embodiments that are not
described in the related art can also be considered as another
object of the present invention. Preferably, such items are
necessary for obtaining advantages that cannot be obtained with the
related art.
Reordering Method
The reordering method of the present invention is a reordering
method of sending data (for example, packets), to which information
(for example, numbers) that indicates the order of the packets is
added, from a target base station to a mobile station, and
rearranging the data (packets) at the mobile station according to
this order information.
In the reordering method of the present invention, the target base
station correlates discriminating information which makes it
possible to discriminate data that is forwarded from a source base
station from data that is not obtained via the source base station,
with data that is sent to the mobile station, and transmits the
discrimination information with the data from the target base
station to the mobile station. Preferably, in performing the
reordering process, the mobile station discriminates the data
forwarded from the base station from the data that is not obtained
via the source base station based on the discrimination
information. Moreover, preferably, the data that is not obtained
via the source base station is data that is received from a gateway
device. Furthermore, preferably, the data that is sent to the
mobile station is in the form of packets.
The reordering method of the present invention, comprises: a step
of forwarding packets, which has not yet been sent to the mobile
station, or data, for which a confirmation response of right
reception has not been received from the mobile station, to a
target base station from a source base station; sending that data
to the mobile station from the target base station, as well as
sending data that is received from a host station to the mobile
station; a step of, in the case of sending data received from the
host station with the order jumped, giving the data information
indicating that the data is jump data, then sending the data from
the target base station to the mobile station; and a step of
deciding the jump data is not the object of reordering and
performing the reordering process on data that is the object of
reordering.
Another reordering method of the present invention comprises: a
step of forwarding data that has not yet been sent to the mobile
station or data for which a confirmation response of proper
reception has not been received from the mobile station to a target
base station from a source base station; a step of sending data
that is forwarded from the source base station before a first set
period of time ends and data received from a host station from the
target base station to the mobile station; a step of discarding
data that is forwarded from the source base station after the first
time period has ended; a step of adding information to data to
specify the data is the last data that is object of reordering from
among data that was received before the first time period ends or
adding the information to other data that is sent immediately after
that data and sending the data from the target base station to the
mobile station and a step of performing reordering process until a
second time period ends, and when the data that contains the
aforementioned information is received, ending the reordering
process even though the second time period has not yet ended.
The reordering method described above, also preferably comprises: a
step of giving packets information indicating that the packets are
jump packets when sending packets to the mobile station that were
received from a gateway with the order jumped and sending the
packets to the mobile station from the target base station; and a
step of deciding those jump packets are not objects of reordering,
and performing the reordering process on packets that are the
objects of reordering at the mobile station.
Communication System
The present invention is a communication system in which data (for
example, packets), to which information that indicates the order of
the data is added, is sent from a target base station to a mobile
station, and the data is rearranged in number order at the mobile
station.
The communication system of the present invention comprises a
mobile station, a source base station that communicates with the
mobile station before a handover, and a target base station that
communicates with the mobile station after a handover; where (1)
the source base station comprises: a buffer that saves data that
was received from a host station; a data transmission unit that
transmits the data that is saved in the buffer to the mobile
station; and a control unit that forwards data that was not sent to
the mobile station before execution of a handover sequence, or data
for which a confirmation response of right reception has not been
received from the mobile station to the target base station; (2)
the target base station comprises: a buffer that saves data that is
received from the source base station during execution of a
handover sequence and data that is received from the host station;
a control unit that performs control so that data that is received
from the source base station is sent preferentially to the mobile
station, and when sending data to the mobile station that is
received from the host station with the order jumped, adds
information to the data to indicate that the data is jump data; and
a transmission unit that transmits data to the mobile station; and
(3) the mobile station comprises: a buffer that saves data that is
received from a base station; and a reordering control unit that
decides the jump data is not an object of reordering, and performs
reordering on data that is the object of reordering.
Another communication system of the present invention comprises: a
mobile station, a source base station that communicates with the
mobile station before a handover, and a target base station that
communicates with the mobile station after a handover; where (1)
the source base station comprises: a buffer that saves data that is
received from a host station; a data transmission unit that
transmits the data that is stored in the buffer to the mobile
station; and a control unit that forwards data that is not sent to
mobile station before execution of a handover sequence, or data for
which a confirmation response of right reception has not been
received from the mobile station, to the target base station; (2)
the target base station comprises: a buffer that saves data that is
received from the source base station during execution of a
handover sequence and data that is received from the host station;
a control unit that performs control to send data that is forwarded
from the source base station before a first set period of time ends
to the mobile station preferentially as the object of reordering,
as well as adds information to data to indicated that the data is
last data received as an object of reordering before the first set
period of time ends, or adds the information to other data that is
transmitted immediately after that data and sends the data from the
target base station to the mobile station, and discards data that
is forwarded from the source base station after the first set
period of time has ended; and a transmission unit that transmits
data to the mobile station; and (3) the mobile station comprises: a
buffer that saves data that is received from the base station; and
a reordering control unit that performs reordering until a second
set period of time ends, and when data that contains information
identifying that data is the last data that is an object of
reordering, is received or when data that contains information
indicating that the last data that is an object of reordering has
already been received is, is received, ends the reordering process
even though the second set period of time has not yet ended.
Base Station/Mobile Station
Another form of the present invention is a base station and a
mobile station that form a first or second communication
system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a drawing explaining a first embodiment of the
invention.
FIG. 2 shows an example of the format of a PDCP PDU packet.
FIG. 3 is a drawing explaining packet processing in the PDCP layer
and RLC layer before a handover sequence.
FIG. 4 is a drawing explaining packet processing in the PDCP layer
and RLC layer during a handover sequence.
FIG. 5 is a drawing showing the construction of a base station.
FIG. 6 is a drawing showing the construction of a mobile
station.
FIG. 7 is a flowchart of the operation of a target base station in
a first embodiment of the invention.
FIG. 8 is a flowchart of the operation of a source base station in
a first embodiment of the invention.
FIG. 9 is a flowchart of the operation of a mobile station in a
first embodiment of the invention.
FIG. 10 is a flowchart of the reordering process by a mobile
station.
FIG. 11 is a drawing explaining a second embodiment of the
invention.
FIG. 12 shows an example of the format of a PDCP PDU packet.
FIG. 13 is a drawing explaining packet processing in the PDCP layer
and RLC layer during a handover (1/2).
FIG. 14 is a drawing explaining packet processing in the PDCP layer
and RLC layer during a handover (2/2).
FIG. 15 is a flowchart of the operation of a target base station in
a second embodiment of the invention.
FIG. 16 is a flowchart of the operation of a mobile station in a
second embodiment of the invention.
FIG. 17 shows an example of attaching PDCP PDU sequence numbers SN
to PDCP SDU data and performing notification via the data plane
(U-plane).
FIG. 18 shows an example of attaching PDCP PDU sequence numbers SN
to PDCP SDU data and performing notification via the U-plane, as
well as performing notification of sequence numbers via the
C-plane.
FIG. 19 is a drawing showing the procedure of the handover sequence
in FIG. 18.
FIG. 20 shows an example when there is absolutely no PDCP PDU data
to forward.
FIG. 21 is a drawing explaining the takeover of communication
during a handover.
FIG. 22 is a drawing explaining a handover in a LTE communication
system.
FIG. 23 is a drawing explaining the handover procedure that is
currently presumed for a LTE communication system.
FIG. 24 is a first drawing explaining the reordering process by a
mobile station.
FIG. 25 is a second drawing explaining the reordering process by a
mobile station.
FIG. 26 is a third drawing explaining the reordering process by a
mobile station.
FIG. 27 is a fourth drawing explaining the reordering process by a
mobile station.
FIG. 28 is a drawing explaining the protocol configuration between
a mobile station and a network.
FIG. 29 is a flowchart of the operation of a source base station
during a handover.
FIG. 30 is a flowchart of the operation of a target base station
during a handover.
FIG. 31 is a flowchart of a mobile station during a handover.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(A) Theory of the Present Invention
With the present invention, the problems described above are solved
by making it possible for a base station and a mobile station to
execute the two following procedures.
Procedure 1: When there is a delay in data forwarding from the
source base station after a handover, the target base station sends
data that has already been received from the host station without
waiting to receive the delayed data (jump transmission) and the
mobile station is made to be able to recognize that the data. In
other words, discrimination information is included in, attached to
or correlated with the data in order to recognize that the data is
jump data, and transmitted with the data or transmitted using a
control channel.
Procedure 2: When it is detected that jump transmission has
occurred, the mobile station decides the jump data is not the
object of reordering and holds it in a buffer, and the mobile
station waits for the data being forwarded from the source base
station to arrive. In other words, the mobile station discriminate
data that is forwarded from the source station from data that is
transmitted without going through the source base station using the
discrimination information and performs reordering of the data that
is forwarded from the source station.
In the conventional method, when data forwarding from the source
base station to the target base station is delayed, the mobile
station must wait transmission of the data that is forwarded from
the host station to the target base station. Therefore, by waiting
until a specified amount of time (Waiting Time) has elapsed for
that data from the source base station to arrive, a problem occurs
in that delay in communication increases and the throughput is
degraded. However, as described above, by performing jump
transmission of data, it is possible to quickly send data that was
received from the host station to the mobile station even when data
transfer (forwarding) from the source base station is delayed, and
in doing so, communication delay is reduced. Therefore, when
compared with the conventional method, the present invention is
able to maintain high-quality communication immediately before and
after a handover.
(B) First Embodiment
FIG. 1 is a drawing explaining a first embodiment of the invention,
and in this embodiment, the case of adding order information to
each packet is explained, however, it is also possible to use data
having a specified size.
Here it is presumed that before a handover, packets n-5 to n are
stored in the source base station 11a, and of these packets,
packets n-5 to n-2 are transmitted to the mobile station 14,
however, packets n-1 and n are not transmitted to the mobile
station 14. For example, the packets n-1 and n arrived at the
source base station 11a after the radio communication line between
the source base station 11a and mobile station 14 was cut, so these
packets n-1 and n could not be transmitted to the mobile station
14. Also, of the packets that were transmitted to the mobile
station 14, it is presumed that the mobile station 14 could not
rightly receive the packets n-5 and n-3 (NACK), however was able to
rightly receive the packets n-4 and n-2 (ACK). Therefore, the
mobile station 14 saves packets n-4 and n-2, but does not save
packets n-5 and n-3.
When a handover occurs in this state, the source base station 11a
transfers (forwards) the packets n-5 and n-3 that could not be
rightly received by the mobile station 14 and the unsent packets
n-1 and n to the target base station 11b. The forwarding of the
packets will be explained below, however, the invention is not
limited to this example.
In addition, after the handover, the host station 12 transmits two
packets m to m+1 that are destined for the mobile station 14 to the
target base station 11b. It is presumed that the transfer
(forwarding) of packets n-5 to n is delayed.
When that the target base station 11b receives packets m and m+1
from the host station 12 before packets n-5, n-3, and n-1 to n are
forwarded from the source base station 11b, the target base station
11b adds jump ID code F to the packets m and m+1 that are received
from the host station 12 and sends those packets to the mobile
station 14 first (jump transmission).
The mobile station 14 saves the packets that were received from the
base station and to which jump ID code F is added in a buffer BF2,
and excludes those packets as objects of the reordering process. In
(A) of FIG. 1, the state is shown in which the mobile station 14
saves packets m and m+1 in the buffer BF2, and saves the packets
n-4 and n-2 that were received before the handover in buffer
BF1.
Next, the target base station 11b transmits the packets n-5, n-3
and n-1 to n that were forwarded from the source base station 11a
to the mobile station 14. The mobile station 14 saves the packets
n-5, n-3 and n-1 to n that were received from the target base
station 11b in the buffer BF1, then executes the reordering process
for these packets that were received after the handover and the
packets n-4 and n-2 that were received before the handover (see (B)
of FIG. 1), and delivers the packets to the upper layer in the
order of continuous sequence numbers.
In a case where the mobile station 14 do not receive packets having
continuous sequence number even when a prescribed time has been
elapsed, the mobile station ends reordering process, and rearranges
the order of the packets that have been already received and
delivers them to the upper layer.
The mobile station 14 then delivers the packets to which the jump
ID code F has been added in order to the upper layer.
During forwarding, a source base station can transfer an entire
packet to a target base station, or can transfer data of just part
of the packet (user data portion). Preferably, order information is
added to the forwarded data.
In addition, in FIG. 1, sequence numbers (order information) m, m+1
are assigned to the packet for which jump transmission is
performed, however, the target base station 11b can assign
arbitrary sequence numbers, can add numbers that overlap the
sequence numbers that are added to the forwarded packets, or can
add numbers that do not overlap.
Jump ID Code
As an example of jump ID code F that is added to packets, a 3-bit
`type` field that is included in the PDCP PDU header is used. In
other words, in that type field, a new type number is defined as
jump ID code F, and that type number is added to a packet for which
jump transmission is performed. FIG. 2 is an example of the PDCP
PDU format, where (A) is an example of format when there is no
header, (B) is an example of format when the PDCP PDU sequence
number is not added, and (C) and (D) are examples of format when
the PDCP PDU sequence number is added. In the format of (C) and
(D), a type field and PID field are defined in the header HD, where
the type field indicates the PDCP PDU type. The PID field is a
field that indicates the header compression type that is used for
the data included in the data portion. In the type field,
`type=000` and `type=001` are already regulated, however, the types
`type=010 to 111` are not regulated and are unused. Therefore,
`type=010` is used as the type number (jump ID code) for
discriminating the PDCP PDU for which jump transmission is
performed.
PDCP Layer and RLC Layer Processing Before and After Handover
Control
FIG. 3 is a drawing explaining the packet processing of the PDCP
layer and RLC layer before a handover. The source base station 11b
stores packets n-5 to n-2 of the PDCP layer in a buffer ((A) of
FIG. 3), and in the RLC layer the packets are divided into a
plurality of data as shown in (B), then RLC layer sequence numbers
I, I+1, I+2, . . . , I+6 are added to the divided data, and the
data (RLC PDU data) are sent to the mobile station 14. Before a
handover, the mobile station performs order control for the PDCP
layer using RLC SDU(PDCP PDU) and PLC PDU in the RLC layer. In (C),
(D) of FIG. 3, the mobile station 14 did not rightly receive the
divided data I, I+4 and I+5, or in other words, packets n-5 and
n-3, however, the mobile station 14 did rightly receive packets n-4
and n-2. Before a handover, the mobile station 14 performs order
control for the PDCP layer using RLC SDU (PDCP PDU) and PLC PDU in
the RLC layer, however, after a handover, the RLC layer is
regulated to be initialized. Therefore, processing for order
control moves to the PDCP layer, and the mobile station 14 executes
the handover sequence in the PDCP layer.
During reordering, the mobile station 14 starts a timer in order to
determine when reordering ends.
The arrival of the packets m and m+1 that are received from the
host station 2 is early, so jump ID code is attached to those
packets m, m+1 and they are sent to the mobile station 14 first
(jump transmission). The mobile station 14 saves the packets that
are received from the base station and to which jump ID code is
attached in a buffer B2 (see FIG. 1), and removes them as objects
of the reordering process. After that, the mobile station 14
executes the reordering process for the packets n-5, n-3, and n-1
to n that were received from the target base station 11b and the
packets n-4 and n-2 that were received before the handover, then
delivers the data to the upper layer.
When the specified time TM of the timer has elapsed, the mobile
station 14 ends the reordering process and sends the packets that
have been received to the upper layer even though there maybe
missing packets.
Window Control
FIG. 4 is a drawing explaining the packet processing of the PDCP
layer and RLC layer after a handover, and shows in detail window
control.
In consideration of the fact that more than the allowable amount of
data arrives, the mobile station 14 internally generates a buffer
size window, and during the reordering process, performs the
following window control. In the example shown in FIG. 4, the left
end of the window WD is initially the number n-5 of the expected
data, and the right end of the window is the number decided
according to the amount of allowable data (window buffer size), and
is taken to be n-2. The mobile station 14 does not apply the window
process to the packets to which jump ID code F has been added.
In the initial state (window state (0)), when the mobile station 14
receives the expected packet (n-5) and executes the reordering
process, the window state becomes as shown in (1). Therefore, the
mobile station 14 delivers the expected packet (n-5) and the packet
(n-4) that continues after the packets(n-5) to the upper layer.
Thereafter, the window state becomes as shown in (2), and in this
state, when the mobile station 14 receives the expected packet
(n-3) and performs the reordering process, the window state becomes
as shown in (3). Therefore, the mobile station 14 delivers the
expected packet (n-3) and the packet (n-2) that continues after the
packet(n-3) to the upper layer. The window state then becomes as
shown in (4), and in this state, the mobile station 14 waits to
receive the packets (n-1) and n, and when the packets are received
within the set period of time T.sub.M, the mobile station 14 then
delivers the packets to the upper layer. However, when the mobile
station 14 does not receive the packets (n-1) and n within the set
period of time T.sub.M, the mobile station 14 delivers the packets
starting from packet m that are saved in the buffer BF2 to the
upper layer, and moves to normal control as before handover. As was
explained above, by removing the jump packets from the reordering
process and performing window control as described above, it is
possible to execute the reordering processing on just packets n-5
to n.
In this window control, when a packet is received that has a
sequence number that is smaller than the sequence number on the
left end of the window, the mobile station 14 deletes that packet.
Also, when a packet is received that has a sequence number that is
larger than the sequence number on the right side of the window,
the mobile station 14 takes the sequence number of that packet to
be the sequence number on the right side of the window and changes
the sequence number on the left side of the window according to the
window buffer size, while at the same time, delivers the packets
that have moved out of the window range and that have been received
to the upper layer.
If this window processing is applied to the jump packets as well,
at the instant that packet m is received, the window state will be
such that the left side of the window is n-2 and the right side of
the window is m. In this case, the mobile station 14 gives up on
receiving the packets n-5 and n-3 that have not yet been received,
and delivers the already received packets n-4 and n-2 to the upper
layer. In addition, the mobile station decides the sequence number
of the right side of the window to m, and decides the sequence
number of the left side of the window to the number n-1 that is
determined according to the window size, then after that waits for
packet n-1, packet n, packet n+1, . . . , packet m-1 to arrive.
However, since packet n+1 to packet m-1 are packets that do not
actually exist, the mobile station uselessly waits to receive those
packets, and trouble occurs in packet processing.
Construction of a Base Station
FIG. 5 is a drawing that shows the construction of a base station,
and shows the buffer unit, scheduler unit, transmission/reception
unit and control unit.
The buffer unit 21 is memory for storing packets that come from the
host station, and packets that are forwarded from an adjacent base
station (source base station). In the FIG. 5, two buffers 21a, 21b
are physically provided, however, construction is also possible in
which just one memory is physically provided, and that one memory
is used by dividing it using software.
The scheduler unit 22 selects a mobile station from among a
plurality of communicating mobile stations with which to perform
radio transmission, fetches packets for that mobile station that
are stored in the buffer unit and sends them to the
transmission/reception unit 23. The transmission/reception unit 23
encodes and modulates the packets that are input from the scheduler
unit 22, and transmits the actual data using radio communication.
In addition, the transmission/reception unit 23 receives and
demodulates control signals and various data that are sent from the
mobile station.
The control unit 24 comprises a buffer management unit 24a, HO
control unit 24b and measurement control unit 24c. The buffer
management unit 24a manages the various packets that are stored in
the buffer 21. When data is taken over in a handover, the control
unit 24 forwards at least the packets that are stored in the buffer
unit 21b for which a confirmation (ACK) indicating that the packets
were rightly received has not been obtained from the mobile station
to the target base station 11b.
On the other hand, when the arrival of packets that are being
forwarded from the source base station 11a are delayed due to the
take over of data, and jump transmission is performed for the
packets that are received from the host station 12, `type=010` is
entered into the type field of the headers of the packets that will
jump.
The HO control unit 24b executes handover control as explained in
FIG. 23, and the measurement control unit 24c gathers various
measurement data that is sent from the mobile station, such as the
radio communication quality CQI (Channel Quality Information) of
the mobile station and the like.
Construction of a Mobile Station
FIG. 6 is a drawing showing the construction of a mobile station,
and shows the transmission/reception unit 31, buffer unit 32,
reordering unit 33 and control unit 34. The transmission/reception
unit 31 transmits packets and control information to or receives
packets and control information from a base station. When RLC PDU
data could not be created from a lower-layer packet that was
received, the buffer unit 32 holds that lower-layer packet until
that RLC PDU data can be created, and after the RLC PDU data is
created, the buffer unit 32 removes the header and delivers that
data to the reordering unit 33 as RLC SDU (PDCP PDU) data. The
reordering unit 33 has a function for rearranging the PDCP PDU data
in order of sequence numbers and delivers the data to the upper
layer. When a missing PDCP PDU sequence number is detected, the
reordering unit 33 saves the PDCP PDU data following that PDCP PDU
data in internal memory until the PDCP PDU data with the continuing
sequence number is received. However, when that PDC PDU data has
still not arrived after a set period of time has elapsed, the
reordering unit 33 stops the reordering process and delivers all of
the stored PDCP PDU data to the upper layer. Moreover, the
reordering unit 33 performs window control so that the amount of
data being processed does not exceed the allowable amount.
The control unit 34 comprises a measurement unit 34a, reordering
management unit 34b and retransmission management unit 34c. The
measurement unit 34a measures various kinds of measurement
information that are sent to a base station. For example, the
measurement unit 34a measures the radio communication quality
(Channel Quality Information) of the mobile station. The reordering
management unit 34b controls the reordering unit 33, and when there
is a missing sequence number in the PDCP PDU data that is held by
the reordering unit 33, the reordering management unit 34b
instructs the reordering unit 33 to wait for that PDCP PDU data
having the continuing sequence number to arrive. Moreover, when a
set amount of time for waiting for that packet to arrive has
elapsed, the reordering management unit 34b instructs the
reordering unit 33 to stop the reordering process, as well as
instructs the reordering unit 33 to delete the headers of all of
the saved PDCP PDU data and to delivers that data to the upper
layer as PDCP SDU data, then sets the reordering unit 33 to a state
in which it is able to receive new PDCP PDU data. In addition, the
reordering management unit 34b finds the maximum sequence number
from among sequence numbers that have been received up to that time
as the sequence number on the right end of the window, and
determines and sets the sequence number of the left end of the
window by taking into considering the window size. Here, in the
case where there is a packet that has already been received that
has a sequence number that is smaller than the sequence number on
the left end of the window, that packet is immediately delivered to
the upper layer. When there is retransmission control, the
retransmission management unit 34c sends a retransmission request
signal to a base station via the transmission/reception unit by a
route indicated by the dashed line.
Operation of the Target Base Station
FIG. 7 is a flowchart showing the operation of the target base
station in a first embodiment of the invention.
When the handover control unit 24b of the target base station 11b
receives a HO request from the source base station 11a (including
the mobile station ID, QoS information, and the like) (step S201),
the handover control unit 24b determines whether to allow accepting
the mobile station (step 202). When accepting the mobile station is
not allowed, the handover control unit 24b performs post processing
(step 213) and ends handover control.
On the other hand, in a case where the handover control unit 24b
allows the acceptance of the mobile station, the handover control
unit 24b return a HO request response message to the source base
station 11a (step 203). Then the target base station waits the
packets that are forwarded from the source base station 11a. When
the packets arrivers from the source base station, the buffer 21
stores them (step 204).
When the handover control unit 24b receives a HO complete report
from the mobile station 14 (step 205), the handover control unit
24b then sends a HO complete report to the host station 12 (step
206). After receiving the HO complete report, the host station 12
changes the transmission path for packets from the source base
station 11a to the target base station 11b, and returns a HO
complete response to the target base station 11b. After the
handover control unit 24b of the target base station 11b receives
the HO complete response from the host station 12 (step 207), the
handover control unit 24b sends an instruction to the scheduler
unit 22 to start transmitting packets (step 208).
From this, the scheduler unit 22 checks whether jump transmission
of packets is necessary (step 209), and when jump transmission is
not necessary, the scheduler unit 22 sends the packets that were
forwarded from the source base station 11a to the mobile station 14
first (step 210). However, when forwarding of packets from the
source base station 11a is delayed, and it is necessary to perform
jump transmission of packets, the scheduler unit 22 executes jump
transmission of the packets that were received from the host
station 12.
In order to execute jump transmission, the scheduler unit 22 sets
the type number in the type fields of the packets for which jump
transmission will be performed to 010 (type=010) in order that the
mobile station 14 will be able to recognize that the packets are
jump packets (step 211), then after that, sends the packets to the
mobile station 14 (step 210).
At the same time as this, the handover control unit 24b sends a
resource release to the source base station 11a (step 212), then
performs post processing (step 213) and ends handover control.
Operation of the Source Base Station
FIG. 8 is a flowchart showing the operation of a source base
station in a first embodiment of the invention.
In FIG. 8, when the measurement control unit 24c of the source base
station 11a receives a Measurement Report which indicates reception
state information from the mobile station 14 (step 251), the
handover control unit 24a determines whether or not a handover (HO)
is necessary based on that reception state information (step 252),
and when a handover is not necessary, returns to the start.
However, when the handover control unit 24b determines that a
handover HO is necessary, the handover control unit 24b decides a
target base station 11b according to the contents of the
Measurement Report and sends a handover request to that target base
station 11b (step 253).
After that, when a HO response message that is sent from the target
base station 11b is received (step 254), the HO control unit 24b
sends a HO instruction message to the mobile station 14 (step 255),
and instructs the buffer management unit 24a to forward packets
that are saved in the buffer 21b to the target base station 11b.
Thereby, using the route indicated by the dotted line, the buffer
management unit 24a forwards packets that are saved in the buffer
21b and that were not sent to the mobile station 14, or packets
that were not received rightly (NACK packets) by the mobile
station, to the target base station 11b (step 256). After that,
when a resource release message is received from the target base
station 11b (step 257), the HO control unit 24a performs a resource
release (step 258).
Operation of the Mobile Station
FIG. 9 is a flowchart showing the operation of the mobile station.
The measurement unit 34a of the mobile station 14 notifies the
source base station 11a of the reception status using a Measurement
Report (step 271). The control unit 34 then waits for a HO
instruction message to be sent from the source base station 11a,
and when a HO instruction message is received (step 272), the
control unit 24 establishes synchronization between the mobile
station and the target base station 11b using L1/L2 signaling (step
273), and after synchronization is established, sends a handover
complete report to the target base station 11b (step 274). After
that, the control unit 34 checks whether a received packet is a
jump packet, or in other words whether the type number of a packet
is type=010 (step 275), and when a packet is a jump packet, the
control unit 34 removes that packet as an object of reordering and
saves the packet in a buffer 32, then after a set period of time
has elapsed, removes the header and delivers the packet to the
upper layer as a PDCP SDU packet (step 276). On the other hand,
when a packet is not a jump packet, the control unit 24 executes
reordering, and rearranges the order according to the sequence
number, then removes to header from the reordered packets and
delivers them to the upper layer as a PDCP SDU packets (step
277).
FIG. 10 is a flowchart showing the reordering process by the mobile
station.
When the transmission/reception unit 31 of the mobile station 31
receives a lower layer packet from the target base station 11b
(step 301), the reordering management unit 34b checks whether it is
possible to create RLC PDU data (step 302), and when it is not
possible to create RLC PDU data, checks whether a set amount of
time has elapsed (step 303), and when the set amount of time has
not elapsed, saves the lower layer packet in the buffer 32 (step
304), then performs processing again from step 301. When it is not
possible to create RLC PDU data even though a set amount of time
has elapsed since the lower layer packet was received, that lower
layer packet is deleted from the buffer (step 305).
On the other hand, in step 302, when it is possible to create RLC
PDU data using the received lower layer packet, that RLC PDU data
is delivered to the reordering unit 33 as RLC SDU (PDCP PDU) data
(step 306). After receiving the RLC SDU (PDCP PDU) data, the
reordering unit 33 checks whether there is a missing sequence
number (step 307), and when there is no missing sequence number and
the sequence numbers are continuous, the reordering unit 33 removes
the header of that RLC SDU (PDCP PDU) data and delivers it to the
upper layer as PDCP SDU data (step 311). However, when there is a
missing sequence number, the reordering management unit 34b
instructs the reordering unit 33 to save the PDCP PDU data (step
308). Thereby, the reordering unit 33 saves the RLC SDU (PDCP PDU)
data, and checks whether RLC SDU (PDCP PDU) data with a continuing
sequence number is received (step 309). After receiving RLC SDU
(PDCP PDU) data with a continuing sequence number, the reordering
unit 33 removes the header of the RLC SDU (PDCP PDU) data and
delivers it to the upper layer as PDCP SDU data, and then delivers
similarly the saved PDCP PDU as PDCP SDU to the upper layer (step
311).
Moreover, in step 309, when RLC SDU (PDCP PDU) data having a
continuing sequence number is not received, the reordering unit 33
monitors a preset period of time T.sub.M (step 310), and when the
set period of time has not yet elapsed, repeats processing from
step 308, however, when the set period of time T.sub.M has elapsed,
the reordering unit 33 removes the header of the saved PDCP PDU
data and delivers it to the upper layer even though the sequence
numbers are not continuous (step 311).
With the first embodiment of the invention as described above,
packets that are transmitted from the host station 12 to the target
base station 11b can be transmitted to the mobile station 14 as
jump packets without waiting, so it is possible to eliminate delay
time of the data, and to improve the throughput of the overall
system. Moreover, the mobile station 14 removes jump packets as
objects of reordering control (sequence order control), and
performs sequence order control on packets other than jump packets
and delivers those packets to the upper layer in order of sequence
numbers. As a result, the mobile station is able to perform
sequence order control of packets even when there is only one order
control function.
(C) Second Embodiment
In the first embodiment, the mobile station 14 performed the
reordering process during a set period of time TM, and ended the
reordering process when that set period of time TM elapsed (see
step 310 in FIG. 10). In that case, when the set period of time TM
has not yet elapsed even though all of the packets that have been
forwarded from the source base station 11a to the target base
station 11b are received, the mobile station 14 continues the
reordering process. Therefore, in a second embodiment of the
invention, in order that the mobile station 14 is able to recognize
the last packet that is an object of the reordering process, the
target base station 11b adds an identifier to a specified packet
(last packet) and transmits that packet to the mobile station 14,
and after receiving that last packet, the mobile station 14
immediately ends the reordering process even though the set time TM
has not elapsed.
FIG. 11 is a drawing explaining a second embodiment of the
invention, where it presumed that packets n-5 to n are stored at
the source base station 11a before a handover, and of these
packets, packets n-5 to n-2 were sent to the mobile station 14,
however, packets n-1 and n were not sent to the mobile station 14.
For example, packets n-1 and n arrived after the radio
communication line between the source base station 11a and the
mobile station had been cut, so these packets n-1, n were not sent
to the mobile station 14. In addition, it is presumed that of the
packets that were sent to the mobile station 14, packets n-5 and
n-3 were not received rightly by the mobile station 14 (NACK), and
that packets n-4 and n-2 were received rightly (ACK). Therefore,
the mobile station 14 saves packets n-4 and n-2, and does not save
packets n-5 and n-3.
When a handover occurs in this state, the source base station 11a
forwards the packets n-5 and n-3 that were not rightly received by
the mobile station 14, and the unsent packets n-1 and n to the
target base station 11b. Also, the host station 12 sends two
packets m to m+1 to the target base station 11b after the handover.
It is presumed that the forwarding of the packets n-5, n-3 and n-1
to n is delayed.
When the target base station 11b receives the packets m and m+1
from the host station 12 before the packets n-5, n-3 and n-1 to n
are forwarded from the source base station 11a, the target base
station 11b adds jump ID code F to the packets m and m+1 that were
received from the host station and sends them to the mobile station
14 first (jump transmission). The mobile station 14 saves the
packets received from the base station with the jump ID code added
in the buffer BF2, and removes them as objects of the reordering
process. In (A) of FIG. 11, the state is shown in which the mobile
station 14 saves packets m and m+1 in the buffer BF2, and saves the
packets n-4 and n-2 that were received before the handover in the
buffer BF1.
The target base station 11b sends the packets that are forwarded
until a preset time T.sub.M (called the Waiting Time) has elapsed
to the mobile station 14, and when the Waiting Time has elapsed the
target base station 11b discards any packets that may be forwarded.
Therefore, the target base station 11b receives the packets n-5,
n-3 and n-1 that were forwarded from the source base station 11a
before the Waiting Time elapsed and saves them in the buffer BF,
then sends the packets one by one to the mobile station 14. In
addition, the target base station 11b receives packet n from the
source base station 11a, however, the Waiting Time was complete
before the packet n could be sent to the mobile station 14. In this
case, the target base station 11b adds an identifier (L) to packet
n to indicate that it is the last packet forwarded from the source
base station, and sends packet n to the mobile station 14.
The mobile station 14 saves the packets n-5, n-3, n-1 and n that
were received from the target base station 11b in the buffer BF1,
and as shown in (B) of FIG. 11, executes the reordering process to
rearrange these packets and the packets n-4 and n-2 that were
received before handover. Moreover, after detecting packet n with
the identifier (L) added, the mobile station 14 determines that
forwarding has ended, so delivers the packets that have been
reordered to the upper layer and ends the reordering process even
though the set time period T.sub.M for reordering has not
ended.
The case in which the Waiting Time T.sub.W ends before packet n is
sent is described above, however, the Waiting Time T.sub.W may end
after the packet n has been sent to the mobile station. In that
case, the target base station 11b adds an identifier (L) to a
packet m+2 that will be received from the host station 12
indicating that it is the last packet, and sends the packet m+2 to
the mobile station 14. After receiving the last packet to which
that identifier (L) has been added, the mobile station 14
determines that forwarding has ended, and immediately ends
reordering even though the set period of time T.sub.M for
reordering has not ended.
Moreover, packets n-5 and n-3 are received by forwarding, however,
the Waiting Time T.sub.W may end in the stage before packets n-1
and n are received. In that case, the target base station 11b may
add an identifier (L) to packet n-3 indicating that it is the last
packet, and sends that packet to the mobile station 14. The mobile
station 14 detects the Last packet to which that identifier (L) is
added and ends the reordering process.
Last Packet ID Code L To add an identifier (last packet ID code) L
to a packet identifying that the packet is the last packet, a 3-bit
`type` field that is included in the PDCP PDU header is used. In
other words, a new type number is defined in that `type` field as
the last packet ID code L, and that type number is assigned to the
first packet to be sent after the Waiting Time T.sub.W ends. FIG.
12 shows an example of the PDCP PDU format, where (A) is an example
of format without a header, (B) is an example of format in which
the PDCP PDU sequence number is not added, and (C) and (D) are
examples of format in which the PDCP PDU sequence number is added.
In the format shown in (C) and (D), a type field and a PID field
are defined in the header HD, where the type field indicates the
type of PDCP PDU. For the type field, `type=000` and `type=001` are
already regulated, however, the type numbers `type=010 to 111` are
not regulated and are unused. Therefore, `type=011` is used as the
type number for recognizing the PDCP PDU packet (last packet) that
is to be sent first after the Waiting Time T.sub.W has ended.
PDCP Layer and RLC Layer Processing
FIG. 13 and FIG. 14 are drawings explaining packet processing for
the PDCP layer and RLC layer after a handover.
FIG. 13 shows the case in which the last packet ID code L is added
to the packet n that is sent to the mobile station 14. In the first
embodiment, during reordering, the mobile station 14 starts a timer
in order to determine the end of reordering. Here, when the set
time T.sub.M is set to be a large value, the reordering process
continues regardless of whether or not packet n is the last packet
forwarded, and the packets cannot be delivered to the upper layer
until the set time T.sub.M ends. However, in this second
embodiment, after receiving packet n (Last packet) to which the
last packet ID code L has been added, the mobile station 14
immediately ends reordering and delivers all of the PDCP PDU
packets to the upper layer.
When more than the allowable amount of data arrives at the mobile
station 14, the mobile station 14 performs window control during
reordering. The left side of the window is n-5, which is the number
expected for arrival, and the right side of the window is the value
of the upper limit of the allowable amount of data (in the figure,
this value is n). However, window processing is not applied to
`type=010` packets (jump packets) as in the first embodiment. By
performing processing in this way, it is possible to execute
reordering from packet n-5 to packet n.
FIG. 14 shows the case in which last packet ID code L is added to
packet m+2. At the instant that packet m+2 (Last packet) is
received, the mobile station 14 immediately ends reordering even
though the set time T.sub.M may not have ended yet, and delivers
all of the received PDCP PDU packets to the upper layer. Also, in
the case of window control, the mobile station 14 executes window
control in the same way as shown in FIG. 13.
Operation of the Target Base Station
FIG. 15 is a flowchart showing the operation of the target base
station in the second embodiment of the invention, where the same
reference numbers are given to steps that are the same as the steps
of the first embodiment shown in FIG. 7. The base stations and
mobile station of this second embodiment have the construction
shown in FIG. 5 and FIG. 6.
When the handover control unit 24b of the target base station 11b
receives a HO request from the source base station 11a (includes
the mobile station ID, QoS information, etc.), the handover control
unit 24b performs call-receiving control based on that information
and determines whether to allow acceptance of the mobile station.
When acceptance is not allowed, the handover control unit 24b
performs post processing, and ends handover control (steps 201 to
202, and 213).
On the other hand, when the handover control unit 24b allows the
acceptance of the mobile station 14, the handover control unit 24b
returns a HO request response message to the source base station
11a, and starts measuring the elapsed time. After that, the target
base station 11b waits for packets forwarded from the source base
station 11a, and stores the packets in the buffer unit 21 (steps
203 and 204).
In this state, after receiving a HO complete report from the mobile
station 14, the handover control unit 24b sends a HO complete
report to the host station 12 (steps 205 and 206). After the host
station 12 receives the handover complete report, the host station
12 then changes the transmission path for the packets from the
source base station 11a to the target base station 11b, and returns
a HO complete response to the target base station 11b. The handover
control unit 24b of the target base station 11b receives the HO
complete response from the host station 12, and then instructs the
scheduler unit 22 to start transmitting packets to the mobile
station(steps 207 and 208).
Next, the scheduler unit 22 monitors whether or not the elapsed
time has exceeded a set period of time T.sub.W, that is, whether or
not the Waiting Time has ended (step 501), and when the Waiting
Time has not ended, checks whether it is necessary to perform jump
transmission of packets (step 209), and when it is not necessary,
starts sending packets that were forwarded from the source base
station 11a (step 210) preferentially to the mobile unit 14 via
transmission/reception unit 23. However, when forwarding is delayed
and it is necessary to perform jump transmission of packets, the
scheduler unit 22 executes jump transmission of packets that are
received from the host station 12. In order to execute jump
transmission, the scheduler unit 22 sets the type number of the
type field in the header of the packets for which jump transmission
will be performed to 010 (type=010), thereby the mobile station 14
will be able to identify that the packets are jump packets (step
211). After that, the transmission/reception unit 23 sends the jump
packets to the mobile station.
At the same time as this, the handover control unit 24b sends a
resource release to the source base station 11a (step 212), then
returns to step 501 to check whether the Waiting Time has ended,
and when the Waiting Time has not ended, repeats the process from
step 209.
On the other hand, in step 501, when the Waiting Time T.sub.W has
ended, the scheduler unit 22 sets the type number of the packet
that will be transmitted immediately after the Waiting Time has
ended to `011`. In other words, the scheduler unit 22 adds a last
packet ID code L to the packet that will be transmitted immediately
after the Waiting Time ends, and transmits that packet (Last
packet) to the mobile station 14 (steps 502 and 503), then after
that, performs post processing and ends handover control (steps 212
and 213).
In step 502, in order to be able to identify whether the last
packet to which the last packet ID code L has been added is a
packet that has been forwarded from the source base station 11a, or
is a packet that was received from the host station 12, the
scheduler unit 22 sets the type number to `011` in the case of the
former, and sets the type number to `100` in the case of the
latter. By doing this, it becomes easy for the mobile station 14 to
perform the reordering process as will be described later.
Operation of the Mobile Station
FIG. 16 is a flowchart showing the operation of the mobile station,
where the same reference numbers are used for steps that are the
same those in the operation flowchart shown in FIG. 98. The
measurement unit 34a of the mobile station 14 uses a Measurement
Report to notify the source base station 11a of the reception
status. The control unit 34 then waits for a HO instruction message
to be sent from the source base station 11a, and after receiving
the HO instruction message, establishes synchronization between the
mobile station 14 and the target base station 11b using L1/L2
signaling, and after synchronization has been established, sends a
handover complete report to the target base station 11b (steps 271
to 274).
After that, the control unit 34 checks whether a received packet is
a jump packet, or in other words, a packet having a type number
type=010 (sep 600), and when the packet is a jump packet, removes
that packet from being an object of reordering and saves it in the
buffer 32 (step 601).
On the other hand, when the packet is not a jump packet, the
control unit 34 checks whether the received packet is the Last
packet, or in other words, a packet having a type number type=011
(sep 602). In the case where the packet is not the Last packet, the
control unit 34 starts the reordering process, and arranges the
PDCP PDU packets according to sequence numbers, then creates PDCP
SDU data and delivers that data to the upper layer (step 603).
The control unit 34 then checks whether the preset time period
T.sub.M has ended (step 604), and when the set time period T.sub.M
has not yet ended, the control unit 34 repeats processing from step
600, and when the set time period T.sub.M has ended, the control
unit 34 ends the reordering process, then deletes the header from
RLC SEU (PDCP PDU) packets that have not yet been delivered to the
upper layer, creates PDCP SDU data and delivers that data to the
upper layer (step 605). Next, the control unit 34 delivers packets
that are not the object of reordering to the upper layer and ends
processing (step 606).
However, in step 602, when the received packet is the Last packet,
the control unit 34 immediately stops the reordering process (step
607). At that time, in the case where the Last packet is a
forwarded packet, the control unit 34 creates a PDCP SDU packet
from that Last packet and delivers that packet to the upper layer,
then stops reordering. When the Last packet is a packet received
from the host station 12, the control unit 34 immediately stops
reordering, and connects the Last packet to the last packet that is
not an object of reordering and stored in the buffer. After that,
the control unit 34 delivers the packets that were not the object
of reordering to the upper layer and ends processing (step
606).
As described above, with this second embodiment of the invention,
the mobile station detects the end of the forwarded packets by
referencing the Last packet ID code, making it possible to quickly
stop the reordering process. Therefore, it is possible to eliminate
the data delay time, and improve the throughput of the overall
system.
(D) Third Embodiment
The order information (sequence numbers n, m) that has been
presented up until now has been convenient numbers for simplifying
the explanation, however, actually, it is information that is added
at the base station.
As was explained above, in a LTE communication system, forwarding
is performed in PDCP SDU data units. Therefore, when forwarding
occurred, it was not possible to send the Sequence Number field
from the source base station 11a to the target base station 11b,
and it was necessary to notify the target base station 11b of the
sequence numbers using some method.
In this embodiment, the source base station 11a notifies the target
base station 11b of the sequence numbers together with the PDCP SDU
data, and based on those sequence numbers, the target base station
11b adds a header including the sequence numbers to the forwarded
PDCP SDU data (packets) thereby PDCP PDU(RLC SDU) packet is
created.
FIG. 17 is an example of sending the PDCP PDU sequence number
together with the PDCP SDU data (packets) via the data plane
(U-plane). In FIG. 17, only the PDCP SDU data and the sequence
numbers that accompany that data are shown, however, in order that
the PDCP SDU data is recognized to be a bunch of packets, control
information (header information) must be added. Each time the
source base station 11a forwards PDCP SDU data, the source base
station 11a notifies the target base station 11b of the sequence
number that accompanies that PDCP SDU data using the format shown
in FIG. 17, and based on that sequence number, the target base
station 11b adds the sequence number to the forwarded PDCP SDU data
(packet).
In other words, in the condition shown in FIG. 17, the source base
station 11a first forwards sequence number n-5 and the first PDCP
SDU data via the data plane U-plane. Next, the source base station
11a forwards sequence number n-3 and the next PDCP SDU data via the
data plane U-plane. After that, when forwarding following PDCP SDU
data that were received from the host station 12, the source base
station 11a similarly forwards the PDCP SDU data and the sequence
numbers n-1 and n thereof to the target base station 11b via the
U-plane. At the target base station 11b, by receiving the sequence
numbers n-5, n-3, n-1 and n that were notified via the U-plane, it
is possible for the target base station 11b to recognize the
numbers as the sequence numbers for the forwarded data.
FIG. 18 is an example of the source base station 11a notifying the
target base station 11b via the U-plane of the PDCP PDU sequence
number together with the appropriate PDCP SDU data, and notifying
the target base station 11b via the C-plane of the sequence number
of PDCP SDU data, for which right reception by the mobile station
could not be confirmed, as the Next SN. Notification of the
sequence number (Next SN) via this C-plane is shown as taking over
of the SN (SN takeover) in the handover sequence shown in FIG. 19.
It may possible to perform SN taking over at the same time that the
source base station 11a sends a HO request to the target base
station 11b.
In the state shown in FIG. 18, the source base station 11a first
forwards the sequence number n-5 and the first PDCP SDU data via
the U-plane. Also, at the same time, the source base station 11a
notifies the target base station via the C-plane of the just the
sequence number n-5 of the PDCP SDU data, for which proper
reception by the mobile station could not be confirmed, as the Next
SN (SN takeover). Next, the source base station 11a forwards the
sequence number n-3 and the next PDCP SDU data via the U-plane.
After that, when forwarding the PDCP SDU data that was received
from the host station 12, the source base station 11a similarly
forwards that PDCP SDU data and the sequence number n-1, n thereof
to the target base station 11b via the U-plane. The target base
station 11b receives the sequence number n-5 that was notified via
the C-plane, and the sequence numbers n-5, n-3, n-1 and n that were
notified via the U-plane, however, since the sequence numbers that
were notified via the U-plane are larger than the sequence number
notified via the C-plane, the target base station 11b adds the
sequence numbers that were notified via the U-plane to the
following PDCP SDU data received via the U-plane, and transmits the
data to the mobile station. In other words, the target base station
11b ignores the sequence number n-5 that was notified via the
C-plane.
FIG. 20 is an example of the case in which there is no PDCP SDU
data to forward.
The source base station 11a notifies the target base station 11b
via the C-plane of the next sequence number n+1, as the Next SN.
When the Waiting Time ends without the target base station 11b
receiving PDCP SDU data from the source base station 11a, the
target base station 11b adds the sequence number to the following
PDCP SDU data based on the sequence number n+1 that was notified
via the C-plane, and transmits the data to the mobile station. In
other words, the source base station 11b sets the sequence number
of the packet m that was received from the host station to n+1 and
transmits that data to the mobile station.
Advantage of the Present Invention
With the present invention described above, it is possible to
quickly send packets that were sent from host station to the target
base station to the mobile station as jump packets, thus making it
possible to eliminate the delay time of data, and to improve the
throughput of the overall system. Moreover, the mobile station
removes jump packets from being the object of reordering control
(sequence order control), and performs order sequence control only
on packets other than jump packets, then delivers the packets to a
higher apparatus (upper layer) in order of sequence numbers. As a
result, the mobile station is able to properly perform sequence
order control of packets that are the object of reordering even
when there is only one sequence order function.
Furthermore, with the present invention, the mobile station detects
the end of the forwarded packets by referencing last packet ID
code, and so is able to quickly stop reordering. Therefore, it is
possible to eliminate the delay time of data, and to improve the
throughput of the overall system.
* * * * *